1. Field of the Invention
The present invention is directed to a charger for a capacitor, which charger supplies electricity to the capacitor, and also a charging method for a charging a capacitor.
2. Related Art
Electrical double layer capacitors have been known as capacitors to store electrical energy. Examples of such electrical double layer capacitors are shown and described in Japanese Patent Laid-Open Publications No. SHO 59-48916, No. HEI 6-140286, and No. HEI 6-302472.
The electric double layer capacitors have been used as power sources for various electrical devices and have been considered for use as power sources for electrical-powered vehicles. To use the electrical double layer capacitors as power sources, it is desirable that they be of a small size. To make them small requires increasing the amount of the electrical energy capacity. An electrical double layer capacitors has electrical characteristics similar to those of a condenser, and its energy capacity E is expressed by 1/2 QV (E=1/2 QV). In this formula, Q represents electric charge (Q), and V represents electric voltages between terminals. According to above mentioned formula, increasing the energy capacity can be achieved by increasing the electrical voltages between terminals. In Japanese Application No. HEI 7-180034 by the present assignee, taking into account this point, an electrical double layer capacitor is disclosed that can be used in higher electrical voltage applications than previously possible. FIG. 1 shows a cross section of an electrical double layer capacitor drawn in the above mentioned application. In FIG. 1, a collector 10 accompanies a polarized positive electrode 14 (called the "positive electrode 14") while a collector 12 accompanies a polarized negative electrode 16 (called the "negative electrode 16"). These positive and negative electrodes 14 and 16 include activated carbon and carbon black, which are caked using polytetrafluoroethylene (PTFE) as a binder. The carbon black is effective in enhancing the conductivity of the polarized electrodes. Each of the collectors 10 and 12 is made of aluminum foil. The positive and negative electrodes 14 and 16 are coated on the aluminum foil as the collectors 10 and 12, and then they are baked after being pressed. A separator 18, which is made of a polyolefin-based porous film, is interposed between the positive and negative electrodes 14 and 16. The positive and negative electrodes 14, 16, and the separator 18 are saturated with electrolyte, which contains one mol/liter of tetraethyl ammonium tetrafluoroborate (Et.sub.4 N.cndot.BF.sub.4) dissolved in a solvent of propylene carbonate.
In conventional electrical double layer capacitors, the capacitance of the positive electrode and the capacitance of the negative electrode are set approximately to the same value. On the other hand, in Japanese Application No. HEI 7-180034, the capacitance of the positive electrode and the capacitance of the negative electrode are set to different values as follows.
Hereinafter, the potential of the positive electrode is used as a standard when the positive and negative electrodes are shorted and completely discharged. The difference between the standard and the electric potential of the positive electrode is set as the "positive potential" and the difference between the standard and the electric potential of the negative electrode is set as the "negative potential." The potential, when an irreversible current is caused in the capacitor, is called the "decomposition potential." This decomposition potential is a characteristic value, which is determined based on the electrochemistry.
During charging, with an increase in the applied voltage as the potential rises over the decomposition potential, an irreversible current is caused due to the electrochemical decomposition of the electrolyte. The generation of the irreversible current causes the performance and lifetime of the capacitor to degrade. Therefore, it is necessary to maintain the positive potential and negative potential below the decomposition potential.
The inventors have noted that the positive and negative electrodes have different decomposition potentials when the irreversible current is caused. Also a ratio of the capacitance of the positive electrode to the capacitance of the negative electrode is set to the same or approximately the same reciprocal ratio of the decomposition potential.